It is known to feed motors with a pulse width modulated voltage of constant amplitude, wherein the pulse width modulation (PWM) determines the prevailing torque or the rotational speed, instead of using a constant amplitude direct voltage. This operation is also known as chopper mode. The frequency of the pulse width modulated pulses is above the audible range to avoid the motor control being audible and also to ensure that, despite the pulsed control, a practically direct current, the magnitude of which is determined by the PWM, flows in the winding of the motor due to the integrating effect.
In a known method of initiating the braking of a motor or to change the direction of rotation, the polarity of the voltages to motor windings are reversed and the motor is suddenly driven in the opposite direction of rotation. Under these conditions, initially the motor continues to run in the former direction of rotation, is braked to zero, and then rotates in the opposite direction of rotation. The following difficulties arise with this type of braking. The ends of the motor windings are connected to the operating voltage or to ground via protective diodes. The diodes protect the connected semiconductor components from destruction by voltage peaks generated in the motor windings. When a motor winding is reverse poled at the beginning of braking, a high current, which is produced by the generator voltage or the EMF generated by the motor and the resistance of the motor winding, flows independently of the control action due to conduction in the protection diodes. This current acts as a braking current due to the rotation of the motor i.e. due to the relative movement between the motor windings and the permanent magnets. Since this current cannot fall below the rated value, the braking effect can, in many cases, be too great. For example, in a video recorder, the capstan motor may be considered as the tape drive or tape advancing mechanism. Hence, if the capstan motor is braked too rapidly, loops of magnetic tape can be formed due to the inertia of the feed spool which may continue to turn.